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QSORT(3) FreeBSD Library Functions Manual QSORT(3)
NAMEqsort, qsort_r, heapsort, mergesort - sort functions
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS#include <stdlib.h>
voidqsort(void*base, size_tnmemb, size_tsize,
int(*compar)(constvoid*,constvoid*));
voidqsort_r(void*base, size_tnmemb, size_tsize, void*thunk,
int(*compar)(void*,constvoid*,constvoid*));
intheapsort(void*base, size_tnmemb, size_tsize,
int(*compar)(constvoid*,constvoid*));
intmergesort(void*base, size_tnmemb, size_tsize,
int(*compar)(constvoid*,constvoid*));
DESCRIPTION
The qsort() function is a modified partition-exchange sort, or quicksort.
The heapsort() function is a modified selection sort. The mergesort()
function is a modified merge sort with exponential search intended for
sorting data with pre-existing order.
The qsort() and heapsort() functions sort an array of nmemb objects, the
initial member of which is pointed to by base. The size of each object
is specified by size. The mergesort() function behaves similarly, but
requires that size be greater than ``sizeof(void *) / 2''.
The contents of the array base are sorted in ascending order according to
a comparison function pointed to by compar, which requires two arguments
pointing to the objects being compared.
The comparison function must return an integer less than, equal to, or
greater than zero if the first argument is considered to be respectively
less than, equal to, or greater than the second.
The qsort_r() function behaves identically to qsort(), except that it
takes an additional argument, thunk, which is passed unchanged as the
first argument to function pointed to compar. This allows the comparison
function to access additional data without using global variables, and
thus qsort_r() is suitable for use in functions which must be reentrant.
The algorithms implemented by qsort(), qsort_r(), and heapsort() are not
stable, that is, if two members compare as equal, their order in the
sorted array is undefined. The mergesort() algorithm is stable.
The qsort() and qsort_r() functions are an implementation of C.A.R.
Hoare's ``quicksort'' algorithm, a variant of partition-exchange sorting;
in particular, see D.E. Knuth's AlgorithmQ.Quicksort takes O N lg N
average time. This implementation uses median selection to avoid its O
N**2 worst-case behavior.
The heapsort() function is an implementation of J.W.J. William's
``heapsort'' algorithm, a variant of selection sorting; in particular,
see D.E. Knuth's AlgorithmH.Heapsort takes O N lg N worst-case time.
Its only advantage over qsort() is that it uses almost no additional
memory; while qsort() does not allocate memory, it is implemented using
recursion.
The function mergesort() requires additional memory of size nmemb*size
bytes; it should be used only when space is not at a premium. The
mergesort() function is optimized for data with pre-existing order; its
worst case time is O N lg N; its best case is O N.
Normally, qsort() is faster than mergesort() is faster than heapsort().
Memory availability and pre-existing order in the data can make this
untrue.
RETURN VALUES
The qsort() and qsort_r() functions return no value.
The heapsort() and mergesort() functions return the value 0 if
successful; otherwise the value -1 is returned and the global variable
errno is set to indicate the error.
EXAMPLES
A sample program that sorts an array of int values in place using
qsort(), and then prints the sorted array to standard output is:
#include <stdio.h>
#include <stdlib.h>
/*
* Custom comparison function that can compare 'int' values through pointers
* passed by qsort(3).
*/
static int
int_compare(const void *p1, const void *p2)
{
int left = *(const int *)p1;
int right = *(const int *)p2;
return ((left > right) - (left < right));
}
/*
* Sort an array of 'int' values and print it to standard output.
*/
int
main(void)
{
int int_array[] = { 4, 5, 9, 3, 0, 1, 7, 2, 8, 6 };
const size_t array_size = sizeof(int_array) / sizeof(int_array[0]);
size_t k;
qsort(&int_array, array_size, sizeof(int_array[0]), int_compare);
for (k = 0; k < array_size; k++)
printf(" %d", int_array[k]);
puts("");
return (EXIT_SUCCESS);
}
COMPATIBILITY
Previous versions of qsort() did not permit the comparison routine itself
to call qsort(3). This is no longer true.
ERRORS
The heapsort() and mergesort() functions succeed unless:
[EINVAL] The size argument is zero, or, the size argument to
mergesort() is less than ``sizeof(void *) / 2''.
[ENOMEM] The heapsort() or mergesort() functions were unable to
allocate memory.
SEE ALSOsort(1), radixsort(3)
Hoare, C.A.R., "Quicksort", TheComputerJournal, 5:1, pp. 10-15, 1962.
Williams, J.W.J, "Heapsort", CommunicationsoftheACM, 7:1, pp. 347-348,
1964.
Knuth, D.E., "Sorting and Searching", TheArtofComputerProgramming,
Vol. 3, pp. 114-123, 145-149, 1968.
McIlroy, P.M., "Optimistic Sorting and Information Theoretic Complexity",
FourthAnnualACM-SIAMSymposiumonDiscreteAlgorithms, January 1992.
Bentley, J.L. and McIlroy, M.D., "Engineering a Sort Function",
Software--PracticeandExperience, Vol. 23(11), pp. 1249-1265,
November 1993.
STANDARDS
The qsort() function conforms to ISO/IEC 9899:1990 (``ISO C90'').
FreeBSD 11.0-PRERELEASE February 20, 2013 FreeBSD 11.0-PRERELEASE